Despite appearances to the contrary, more than half of normal-weight Americans have a high percentage of body fat. And, like their overweight contemporaries, this makes them susceptible to heart disease, diabetes and other metabolic disorders, a new study says.

"Body weight is a very blunt instrument; it is not a reliable gauge of obesity, or health, at the individual level," said Dr. David L. Katz, director of the Prevention Research Center at Yale University School of Medicine. "For example, a muscular man may have a very high body weight, yet be perfectly fit and healthy. Many people whose body weight is in the normal range are anything but."

And some people are vulnerable to weight gain in all the wrong places, such as in and around the vital organs of the abdomen, notably the liver, Katz said.

"Even a small amount of extra fat where it matters most can wreak metabolic havoc, increasing risk for diabetes and heart disease, while leaving you with a body weight that looks perfectly innocent," Katz said. "Excess body fat in the belly is a menace, whatever your weight. This study should sensitize patients and providers alike to this concern."